1. Field of the Invention
The present invention pertains to the deposition of alloy films, and more particularly to the formation of alloy films onto cooled base materials during sputter deposition processes.
2. Description of the Prior Art
Metal alloy films are used in many applications including integrated circuit fabrication. These alloy films are often formed by sputter deposition processes onto base materials, such as semiconductor substrates, glass or other materials.
In advanced integrated circuit fabrication, aluminum-based alloy films are commonly used to form metallization interconnects. These aluminum-based alloy films are deposited onto a wafer (substrate) using a sputter deposition process and then etched using known techniques to define the metallization interconnects. During the sputter deposition process, an aluminum-based target is bombarded with ions to form a vapor (of roughly the same composition as the target) which is then transported to the wafer upon which the vapor is deposited to form the thin film. The target composition primarily determines the deposited thin film composition. Using conventional sputter deposition processes, the vapor typically reaches a temperature of 250.degree. to 430.degree. C., and the wafer is maintained at approximately this temperature by a heated gas which is directed onto the wafer.
Aluminum-copper (Al--Cu) alloys are generally favored over other aluminum-based alloys for the formation of metallization interconnects in advanced integrated circuits. Copper is added to the aluminum base material to increase the reliability of the interconnects against electromigration-induced damage. It is generally understood that the addition of copper to pure aluminum or to other aluminum-based alloys (such as aluminum-silicon, aluminum-titanium, and aluminum-magnesium) results in significant improvements in the resistance to electromigration-induced damage, and therefore increases the reliability of the aluminum-based interconnect structure.
A detailed understanding of the basic mechanism for the improvement of electromigration reliability, and the role copper additions play in this improvement, is still unclear and is a matter of scientific inquiry. Therefore, the reliability improvement provided by copper additions cannot be simply summarized, and cannot be generally described by a "universal improvement factor". However, it is roughly generally true and accepted that, for a given overall interconnect structure and device technology, increased copper concentrations lead to increased levels of reliability. Therefore, the increasing reliability demands of advanced integrated circuit technologies require the development of interconnect structures with increased copper concentrations.
The composition of Al-Cu alloys presently used for metallization interconnects typically ranges from 99.5% aluminum-0.5% copper to 99.0% aluminum-1.0% copper, where the concentrations are listed as weight percentages. It is even more desirable to increase the concentration of copper above 1%.
However, problems arise when Al-Cu alloys having 1% copper or more are used in conventional sputter deposition processes. In particular, the deposited aluminum-copper alloy film is comprised of several phases. One phase, the equilibrium intermetallic compound phase "CuAl.sub.2 ", precipitates during deposition and co-exists with the aluminum rich "matrix" phase which forms the basis of the film. These CuAl.sub.2 phases are more difficult to remove during reactive-ion-etching ("RIE") processes, which are used to define and pattern interconnect structures to form the metallization interconnects. After the RIE process, the CuAl.sub.2 phase residues often remain on the surface of the silicon wafer in regions which should normally be cleared of any traces of the aluminum-copper film. These remaining CuAl.sub.2 phase residues present obstacles to defect-free integrated circuit manufacturing and are identified as the source of decreased manufacturing yield during fabrication.
The physical basis for the difficulty in RIE removal of the aluminum-copper films and the CuAl.sub.2 phases is not completely understood. However it is generally believed that, in contrast to aluminum, copper does not form copper-halide compounds of sufficient vapor pressure or volatility during the typical conditions of RIE. As a result the removal rate ("etch rate") of copper rich regions of aluminum-copper films is quite low with respect to the nominally pure aluminum regions. Therefore, local regions of high copper concentration (such as the CuAl.sub.2 phases intermixed throughout the aluminum-copper film) are not removed from the surface of the wafer during RIE, resulting in the observed copper-rich residues.
Multilevel integrated circuit device technologies typically require increased processing temperatures during aluminum-copper alloy sputter deposition processes. Generally, increased aluminum-copper deposition temperatures are required due to the increased severity of the topology of the various layers upon which the aluminum-copper layers are deposited. The increased temperatures during sputter depositions provide increased uniformity of the deposited layer thickness ("step coverage") over the inclined surface topologies, resulting in increased interconnect thickness uniformity over these topographies. However, these increased sputter deposition temperatures of aluminum-copper alloys typically result in increased CuAl.sub.2 precipitation during deposition, resulting in greater manufacturing difficulty and decreased manufacturing yield. In addition, increased copper additions, as desired for the greater device reliability for future device technologies, will lead to further increased CuAl.sub.2 precipitation during elevated temperature depositions, again complicating manufacturing processes and decreasing yields.
Recent advances in "planarization" manufacturing methods and processes have resulted in nearly flat surface topographies. However, the use of conventional elevated-temperature sputter deposition processes has continued for the formation of Al-Cu alloy films.